The emergence of a draft DUP-Tory agreement from more than two years ago makes clear that the parties’ very public relationship this week is the culmination of much preparatory work, not some shotgun marriage.

The document – marked ‘official – sensitive’, suggesting that it was in the Number 10 machine rather than CCHQ – is headed ‘Draft agreement between the Conservative Party and the Democratic Unionist Party’ and dates from the days after the 2015 general election.

Prior to that election, the DUP had been hoping for a hung Parliament but it was assumed at the time that because David Cameron secured an overall majority the party’s influence would be limited to later in the Parliament if the Tory majority was to be eroded.

However, this document shows that Mr Cameron wanted to bolster his majority of 15 right from the start as was prepared to do a deal with the DUP in order to do so.

The document, revealed by the Daily Telegraph yesterday, is significant for a number of reasons.

First, it long predates Theresa May’s time in Number 10 and shows that what she is doing is entirely in keeping with the trajectory on which the party has been in recent years as it has worked more closely with the DUP.

It is now more difficult for Cameroons such as George Osborne to criticise the current arrangement with the DUP in principle, given that their government was seeking something similar – even when it wasn’t necessary to govern.

Secondly, it shows that the deal then being considered went far beyond the ‘confidence and supply’ arrangement which the DUP had wanted going into that election and which is now being discussed.

Instead, the document makes clear that “with the exceptions listed below, DUP MPs will vote with the government on all other matters”. The three exclusions applied to welfare policy, matters relating exclusively to Northern Ireland and issues relating to the devolution of powers to the administrations in Belfast, Cardiff and Edinburgh.

That would have effectively meant DUP MPs agreeing to take the Conservative whip in every vote other than in the three excluded areas.

Thirdly, the document shows us the sort of things which at that point – in a far less perilous situation than that in which Mrs May finds herself – the government was prepared to give in return.

The government would “examine seriously” changes to corporation tax in Northern Ireland so long as the other Northern Ireland parties and the European Commission agreed, would discuss with the DUP “other tax changes”, would agree with the DUP measures to enhance Foreign Office support for outside investment in Northern Ireland, would ensure Northern Ireland got its “fair share” of government contracts and infrastructure investments, would enhance transport and communications links between GB and Northern Ireland, would work to cut electricity costs, and would discuss with the DUP the implementation of the military covenant in Northern Ireland.

Many of those are couched in vague discursive terms rather than as firm commitments, something which almost certainly has changed in this negotiation, given the strength of the DUP position after last Thursday.

Fourthly, the document points to a key consideration of the DUP’s in how it is likely to present this week’s deal. Mindful of the reaction of middle England, the party will be cautious about simply being seen to crudely ask for more public money for a UK region which is already more heavily subsidised by UK taxpayers than England, Scotland or Wales.

On top of the parochial shopping list, the parties would have agreed to a series of national commitments around protecting the foreign aid budget, the size of the diplomatic corps, the current size of the regular armed service along with an increase of 30,000 in the reserves, with “the ambition of holding defence spending at 2% of GDP”.

That is in line with the DUP manifesto for this election and also what Arlene Foster said yesterday in Downing Street, where she referred to national issues such as counter-terrorism as forming part of the DUP-Conservative discussions.

The Daily Telegraph reported that the document was drawn up after secret talks between the DUP and representatives of Mr Cameron and Mr Osborne.

But there are several things which are not yet clear.

Was the document proposed by the Tories or by the DUP? Why was it never agreed? Was there another less formal and perhaps unwritten agreement between the DUP and the Tories in the last Parliament?

Although it is clear that Theresa May is doing this deal out of necessity, it is becoming equally clear that her party – even though many of its MPs appear to have been oblivious to it – has for a long time been preparing the ground for this sort of moment.

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